Spark plug and the method of manufacturing the same

ABSTRACT

A spark plug for internal combustion engines provided with a noble metal tip on the pointed top of a spark plug center electrode. The noble metal tip is welded to the center electrode top end by heating and pressing the portion to be welded. After the welding, the center electrode is shaped to provide the pointed top. The noble metal tip is a platinum alloy consisting of platinum of 85 through 70 in weight percentage and iridium of 15 through 30 in weight percentage.

This is a continuation, of application Ser. No. 372,148, filed Apr. 27,1982, abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a spark plug for an internal combustionengine and a method for manufacturing the same, and more particularly toa spark plug provided with a noble metal tip on the center electrode topend opposing the ground electrode and a method for manufacturing thesame having such a center electrode.

Spark plugs used to ignite a combustible mixture supplied to internalcombustion engines are subjected, during long use, to electrode wearwhich degrades the spark discharge characteristics of the spark plug. Ithas been suggested that the spark plug be provided with a noble metalsuch as platinum on the top end of the center electrode where the sparkdischarge is generated to thereby enhance the durability and hencemaintain the desired spark discharge characteristics.

One type of spark plug suggested in Japanese Utility Model publicationNo. 53-38046 and Japanese Patent Laid-open No. 51-66945, for example, isprovided with a noble metal tip plugged up in a recess drilled axiallyin the center electrode top end opposing the ground electrode. Thisspark plug requires a drilling process to form the recess and requires aconsiderable quantity of noble metal tip to fill the recess to theextent that the noble metal tip is free of attrition or the like, thusresulting in impracticality for commercial production with respect tomanufacturing cost.

The other type of spark plug suggested in Japanese Utility ModelLaid-open No. 54-92227, for example, is provided with a noble metalcoating over the center electrode top end. The bonding between thecenter electrode and the coating is not sufficient to prevent attritionof the coated noble metal caused during long use by the heatdeterioration of the noble metal and the oxidization of the electrodematerial, thus also resulting in impracticality for commercialproduction with respect to product durability.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide aspark plug and a method of manufacture therefor which improve the abovedrawbacks.

It is a further object of the present invention to provide a spark plugand a method of manufacture therefor which are more practical forcommercial production with regard to manufacturing cost and productdurability.

It is a still further object of the present invention to provide a sparkplug and a method of manufacture therefor which use a lesser quantity ofnoble metal but assures sufficient bonding between the center electrodeand the noble metal so that the spark plug can be used for a longerperiod of time without electrode wear.

According to the present invention, a noble metal shaped in a thin disktip is welded to the center electrode top end by resistance welding.Then the top end portion of the center electrode is cutcircumferentially to provide the center electrode with a pointed top onwhich the noble metal remains welded. The noble metal tip welded to thecenter electrode top end is a platinum alloy including iridium of 15through 30 in weight percentage.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. 1 is a side view showing, partly in cross section, a spark plugaccording to a first embodiment of invention;

FIG. 2 is an enlarged view showing, in cross section, a part of thespark plug shown in FIG. 1;

FIGS. 3 through 5 are side views showing the process of manufacturingthe center electrode of the spark plug shown in FIGS. 1 and 2;

FIG. 6 is a side view showing the center electrode having a pointed top;

FIG. 7 is a chart showing the relation between the welding current andthe bonding surface between the center electrode and the noble metaltip;

FIG. 8 is a chart showing the relation between the bonding area of thenoble metal tip to the center electrode and the configuration of thecenter electrode pointed top determined in terms of the length anddiameter thereof; and

FIG. 9 is an enlarged view, showing in cross section a part of the sparkplug according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIGS. 1 and 2 showing a first embodiment of thepresent invention, a center electrode 1, made of a base metal includingnickel, chrome and other elements (as known in the art), and which iselectrically conductive, heat resistant and corrosion resistant issecurely encased and supported in an insulator 3. As shown in FIG. 2,the center electrode 1 is formed with a pointed top 1b of small diameterwhich extends from a trunk 1a of large diameter through a taper 1c. Thepointed top 1b assures good spark discharge characteristics, and thetaper 1c promotes the propagation of flame generated by the sparkdischarge. A noble metal 2, such as platinum shaped in a small diskwhich is 0.7 mm in diameter and 0.3 mm in thickness, for example, iswelded to the top end of the center electrode pointed top 1b. Anelectrically conductive carbon rod 4 is inserted in the axial throughhole 3a of the insulator 3. An electrically conductive brass terminal 5is in threaded engagement with the carbon rod 4. A cylindrical housing 6made of an electrically conductive, heat resistant and corrosionresistant metal securely encases the bottom half of the insulator 3therein, through an airtight ring packing 7 and a caulking ring 8. Thehousing 6 has a thread portion 6a to be threaded to the cylinder head ofan internal combustion engine (not shown). A ground electrode 9 made ofthe like base metal as the center electrode 1 is welded at one endthereof to the bottom end of the housing 6 to be electrically conductivetherewith and is bent to face the pointed top 1b of the center electrode1 leaving an air gap therebetween. The electrodes 1 and 9 areelectrically insulated by the insulator 3 and the air gap. Anelectrically conductive glass seal 10 which is a mixture of copperpowder and glass having a low melting point (as known in the art) isfilled in the through hole 3a of the insulator 3 to electrically connectthe center electrode 1 with the carbon rod 4 and to securely support thesame within the insulator 3.

The spark plug, and the center electrode in particular, described withreference to FIGS. 1 and 2, is manufactured in the following manner. Thebase metal of the center electrode 1 of a uniform diameter is preparedfirst as shown in FIG. 3. Next, as shown in FIG. 4, the noble metal tip2 is welded to the flat end surface of the center electrode byresistance welding by flowing a sufficient electric welding currentunder the condition where the portion to be welded are kept heated andpressed together so that the noble metal tip 2 is completely welded.Then the top end portion of the center electrode 1 is circumferentiallycut to shape the pointed top lb and the taper 1c as shown in FIG. 5. Thepointed top 1b may be formed alternatively by pressing, cold forging orthe like after the welding of the noble metal tip 2 to the top end ofthe center electrode 1. The diameter of the pointed top 1b is preferablykept larger than that of the welded noble metal tip 2 so that noexpensive noble metal is wasted. The center electrode 1 thusmanufactured is assembled with other elements as shown in FIG. 1 in awell know manner.

It is to be noted that, in order to prevent the welded noble metal tip 2from attrition or the like caused by oxidization and corrosion of thewelded portion which is subjected to the high combustion pressure andtemperature of the internal combustion engine, the noble metal tip 2must be welded completely to the center electrode 1. That is, the noblemetal tip 2 must have sufficient welded area all over the surface toprovide a strong bonding with the center electrode 1. For this purpose,it is desired to supply a higher electric welding current to the centerelectrode 1 and the noble metal tip 2 during the resistance welding.Since the higher electric current generated higher Joule heat to softenthe center electrode 1, the center electrode 1 to which the pressingforce is applied during the welding tends to buckle, thus disabling theelectric welding current to flow sufficiently and the noble metal tip 2to be welded completely to the center electrode 1. Therefore, thediameter of the top end of the center electrode 1 to which the noblemetal tip 2 is welded should be as large as possible to prevent thebuckling thereof. Thus it should be understood that the manufacturingmethod described hereinabove with reference to FIGS. 3 through 5 is moreadvantageous in assuring a complete welding of the noble metal tip 2 tothe center electrode 1 than that in which the noble metal tip 2 iswelded to the center electrode 1 after the pointed top lb is shaped asshown in FIG. 6.

The results of an experiment conducted to investigate the sufficiency ofbonding between the center electrode 1 and the noble metal tip 2 isshown in FIG. 7 in which the abscissa and the ordinate indicate,respectively, the electric welding current in amperes and the weldedarea of the noble metal tip 2 in percentage. In the experiment, aplatinum disk of 0.7 mm in diameter and 0.3 mm in thickness was weldedto two kinds of center electrodes, the base metal, Ni-2Cr-3Mn-1.8Si, ofwhich were the same in alloy composition but shaped differently as shownin FIGS. 3 and 6. The a.c. electric welding current was applied duringthe interval of 0.17 sec. under a pressing force of 28 kg. As can beseen from FIG. 7, the platinum tip could be completely welded to thecenter electrode 1 which, as shown in FIG. 3, had a large diameter of22.5 mm at the top end, while it could not be sufficiently welded to thecenter electrode 1 which, as shown in FIG. 6, had a pointed top of 1.5mm in diameter and 0.4 mm in length. The cross mark in FIG. 7 indicatesthe point at which the center electrode 1 began to buckle. The centerelectrodes shaped as shown in FIGS. 3 and 6 began to buckle after andbefore the platinum tip was completely welded, respectively.

A further result of the experiment is shown in FIG. 8 in which theabscissa and the ordinate indicate, respectively, the diameter D and thelength L of the pointed top of the center electrode shaped prior to thewelding of the platinum tip as shown in FIG. 6. The hatched region inFIG. 8 shows the condition where the welding of the platinum tip to thecenter electrode top end was incomplete, that is, the center electrodetop end began buckling before the platinum tip was completely welded tothe center electrode top end.

From these experiments, it should be clear that, in order to assure thecomplete welding of the noble metal tip 2 to the top end of the centerelectrode, the welding of the noble metal tip 2 to the top end of thecenter electrode 1 need be performed prior to the shaping of the pointedtop 1b.

Reference is made next to FIG. 9 which shows a second embodiment of thepresent invention. The spark plug according to the second embodiment,although manufactured in the same manner as that of the firstembodiment, primarily differs therefrom in that, besides the noble metaltip 2 which is 0.7 mm in diameter and 0.3 mm in thickness and welded tothe center electrode top end 1b, a further noble metal tip 11 which is1.2 mm in diameter and 0.2 through 0.3 mm in thickness is welded byresistance welding to the ground electrode 9 to face the noble metal tip2. The ground electrode 9 maintains a uniform thickness and width, sinceno shaping is performed thereon after the welding of the noble metal tip11. The noble metal tips 2 and 11 are a platinum alloy consistingessentially of platinum, 85 through 70 in weight percentage, andiridium, 15 through 30 in weight percentage.

The attrition or coming off of the noble metal tip from the electrodebase metal generally results from oxidization and corrosion of thewelded portion through cracks which are caused axially from the top endsurface of the tip and radially from the circumferential surface of thetip. The radial cracking may be lessened by increasing the thickness ofthe noble metal tip by several hundred microns, which is preferablylimited to 0.5 mm in view of the durability and the manufacturing cost.On the other hand, axial cracking does not deepend on the tip shape butdepends on the composition of the noble metal tip. Therefore, thecomposition of the noble metal tip must be determined so that the noblemetal tip causes the least cracking.

An experiment was conducted to investigate the development of the axialcracking, wherein each platinum alloy tip is subjected repeatedly, twohundred times, to a heat-and-cold cycle test in which the platinum alloytip is kept first at 850° C. for six minutes and then at roomtemperature for six minutes. The results of this experiment are shown inthe following table, wherein Pt, Rh and Ir indicates platinum, rhodiumand iridium, respectively.

    ______________________________________                                        Composition           Axial Cracking                                          ______________________________________                                        100 wt. % Pt          Largest                                                 95 wt. % Pt - 5 wt % Rh                                                                             Largest                                                 90 wt. % Pt - 10 wt. % Rh                                                                           Largest                                                 85 wt. % Pt - 15 wt. % Rh                                                                           Largest                                                 80 wt. % Pt - 20 wt. % Rh                                                                           Large                                                   75 wt. % Pt - 25 wt. % Rh                                                                           Large                                                   70 wt. % Pt - 30 wt. % Rh                                                                           Large                                                   95 wt. % Pt - 5 wt. % Rh                                                                            Largest                                                 90 wt. % Pt - 10 wt. % Ir                                                                           Large                                                   85 wt. % Pt - 15 wt. % Ir                                                                           Little                                                  80 wt. % Pt - 20 wt. % Ir                                                                           Least                                                   75 wt. % Pt - 25 wt. % Ir                                                                           Least                                                   70 wt. % Pt - 30 wt. % Ir                                                                           Least                                                   ______________________________________                                    

The maximum weight percentage of iridium to be included in the platinumalloy must be limited to, preferably 30 in weight percentage from thefact that iridium is more expensive and harder than platinum. If toomuch iridium is to be included, a sheet metal of platinum alloy which ispressed to produce the platinum alloy tip becomes too hard to be pressedwith ease.

It should be understood that the platinum alloy including the platinum,85 through 70 in weight percentage, and the iridium, 15 through 30 inweight percentage is preferable, and that the platinum alloy includingthe platinum, 80 through 70 in weight percentage, and the iridium, 20through 30 in weight percentage, is more preferable.

It is to be pointed out here that the same experimental result asreported in the foregoing table was obtained in another experiment inwhich spark plugs provided with respective platinum alloy tips were usedto run an internal combustion engine (4 cylinder, 4 stroke, 1,600 cc) at5,000 r.p.m. with a full-open throttle for 100 hours. It is to bepointed out further that the platinum alloy tip 2 of 0.9 mm in diameterand 0.4 mm in thickness and the platinum alloy tip 11 of 0.7 mm indiameter and 0.3 mm in thickness, each including iridium of 20 weightpercentage, has presented excellent crackfree characteristics.

The present invention described hereinabove is not limited to the aboveembodiments but may be modified so that the platinum alloy tips 2 and 11include one or more metals selected from silver, gold, palladium,ruthenium and osmium in addition to iridium of 15 through 30 in weightpercentage, the center electrode and the ground electrode are made ofINCONEL 600, available from INCO Company in Canada, which is veryresistant to oxidization and corrosion, and the ground electrode 9 isprovided with no noble metal tip. As should be apparent, the work `top `is arbitrarily used herein to designate one axial end of the spark plugwithout requiring any particular spatial orientation of the spark plugwhen in use.

What is claimed is:
 1. A spark plug for internal combustion enginescomprising:a ground electrode made of an electrically conductive basemetal; a center electrode made of an electrically conductive base metaland provided with a noble metal tip only on a flat end of a tapered endportion thereof, said noble metal tip facing said ground electrodethrough an air gap said noble metal tip being completely resistancewelded under mechanical pressure to said electrically conductive basemetal to provide a strong bond therebetween; and an insulator securelyencasing said center electrode and electrically insulating said centerelectrode from said ground electrode, wherein said noble metal tip ismade of a platinum alloy consisting essentially of 70-80 percentplatinum, by weight, and 30-20 percent iridium, by weight, and whereinsaid center electrode is manufactured by the steps of:preparing the basemetal of said central electrode having a uniform diameter; welding saidnoble metal tip to the top end surface of said center electrode; andshaping said center electrode to provide said flat end of said taperedend portion to be smaller in diameter than said center electrode axiallybeyond said tapered end portion.
 2. A spark plug according to claim 1,wherein said welding step is performed by heating and pressing theportion to be welded.
 3. A spark plug according to claim 1, wherein saidcenter electrode is provided by said shaping step with a tapered endportion which gradually reduces the diameter thereof toward said flatend.
 4. A spark plug according to claim 3, wherein said ground electrodeis provided with a further noble tip metal facing said noble metal tipof said center electrode.
 5. A spark plug according to claim 4, whereinsaid noble metal tips are less than 0.5 mm in thickness and consist ofplatinum of 80 through 70 in weight percentage and iridium of 20 through30 in weight percentage.
 6. A spark plug for internal combustion enginescomprising:a ground electrode made of an electrically conductive basemetal; a center electrode made of an electrically conductive base metaland provided with a flat end of a tapered end portion which is smallerin diameter than said center electrode axially beyond said tapered endportion and which flat end faces said ground electrode; an insulatorsecurely encasing said center electrode and electrically insulating thesame from said ground electrode; and a platinum alloy tip welded only tosaid flat end of said center electrode tapered end portion andconsisting essentially of 70-80 percent platinum, by weight, and 30-20percent iridium.
 7. A spark plug according to claim 6, wherein saidcenter electrode is provided with a tapered end portion which graduallyreduces the diameter thereof toward said flat end, and wherein saidplatinum alloy tip is less than 0.5 mm in thickness.
 8. A spark plugaccording to claim 6 or 7, further comprising:a platinum alloy tipwelded to said ground electrode to face said platinum alloy tip on saidcenter electrode flat end, said platinum alloy tip on said groundelectrode including iridium of at least 15 percent by weight but lessthat 30 percent by weight and less than 0.5 mm in thickness.
 9. A sparkplug according to claim 8, wherein said platinum alloy tip on saidground electrode includes iridium of 20 through 30 in weight percentage.10. A spark plug according to claim 8, wherein said platinum alloy tipon said ground electrode consists of platinum of 85 through 70 in weightpercentage and iridium of 15 through 30 in weight percentage.
 11. Aspark plug according to claim 8, wherein said platinum alloy tip on saidground electrode consists of platinum of 80 through 70 in weightpercentage and iridium of 20 through 30 in weight percentage. .Iadd. 12.A spark plug according to claim 1, wherein a diameter of the top endsurface of the center electrode is larger than the diameter of the noblemetal tip so that after the welding step, the noble metal tip is buriedinside the flat top end surface of the center electrode..Iaddend..Iadd.13. A spark plug according to claim 1, wherein the noblemetal tip has a first surface and a second surface, the first surface ofthe noble metal tip is welded to the top end surface of the centerelectrode and the second surface of the noble metal tip faces the groundelectrode so that a spark gap is formed between the second surface ofthe noble metal tip and the ground electrode. .Iaddend..Iadd.14. A sparkplug according to claim 6, wherein a diameter of the top end surface ofthe center electrode is larger than the diameter of the platinum alloytip so that after the welding step, he platinum alloy tip is buriedinside the flat top end surface of the center electrode..Iaddend..Iadd.15. A spark plug according to claim 6, wherein theplatinum alloy tip has a first surface and a second surface, the firstsurface of the platinum alloy tip is welded to the end of the centerelectrode and the second surface of the platinum alloy tip faces theground electrode so that a spark gap is formed between the secondsurface of the platinum alloy tip and the ground electrode. .Iaddend.